Power modules aimed at railways, vehicles, and industrial machinery that require high voltage and large current operation are provided with ceramic circuit boards having semiconductor elements bonded thereon. To facilitate output from ceramic circuit boards to the exterior, it is common to provide copper electrodes such as to protrude to the outside of ceramic circuit boards.
Traditionally, solder is used to bond ceramic circuit boards and copper electrodes, but there is the problem that the heat and vibrations generated when they are used as power modules cause solder to crack, lowering reliability. Consequently, it has become common to use ultrasonic bonding to directly bond metal circuit boards and copper electrodes. Ultrasonic bonding is a technique for integrating the copper plates and copper electrodes of circuit boards by applying ultrasonic vibrations in the lateral direction while copper electrodes impart a vertical load on ceramic circuit boards.
Aluminum nitride sintered bodies and silicon nitride sintered bodies having high thermal conductivity are being used as the ceramic substrates employed in ceramic circuit boards in response to the increases in the amount of heat generated that accompany the increasing output and integration of semiconductor elements. In particular, as aluminum nitride substrates have higher thermal conductivity compared with silicon nitride substrates, they are suitable as ceramic circuit boards for mounting electronic components with high heat radiation.
However, the converse of aluminum nitride substrates having high thermal conductivity is that they have low mechanical strength, toughness, etc., so there is the problem of cracks forming in the aluminum nitride board surfaces immediately below the portions bonded via vibration during ultrasonic bonding, resulting in the loss of module reliability. In response to this, the following solution was proposed in order to prevent the generation of cracks in ceramic substrates.
Patent Document 1 provides a way to prevent the occurrence of cracks in ceramic substrates by optimizing the positions at which copper electrodes are bonded using ultrasonic bonding and the joint strength between the copper electrodes and metal circuit boards.
Patent Document 1 JP 2002-164461 A